The influence of topography and approach angles on local deflections of airflow within a coastal blowout

Abstract

The spatial variability of air flow through complex topography is an important, but not fully understood, component of dune development and dynamics. This study examines the spatial variability of the wind field in a linear blowout in coastal dunes at Jockey's Ridge State Park, on the Outer Banks of North Carolina. A spatial array of single‐height anemometers and wind vanes were placed within the blowout. Topography exerted a significant steering effect when onshore winds approached from directions within 50° of the blowout axis. Under those conditions wind flow in the blowout aligned to the axis regardless of approach angle, maximizing the potential for erosion and transport in the trough. In other locations aspect variations caused deflection both proportional and disproportional to changes in the approaching wind. When prevailing winds approached from directions more oblique than 50° to the blowout axis, topographic steering through the blowout trough was reduced and secondary flow generated by flow separation over the trough became more prominent. During those approach angles, wind directions and speeds within the upper blowout trough became erratic as vortices and turbulence dominated the flow, minimizing transport potential. The changing characteristics of airflow in the blowout relative to differing approach angles has implications on dune development and variations in transport potential under changing conditions. Copyright © 2013 John Wiley & Sons, Ltd.

Airflow and sediment movement within an inland blowout in Hulun Buir sandy grassland, Inner Mongolia, China

“…Garès and Pease (2015) demonstrated that the directional flow pattern becomes more complex at more oblique approach angles; however, we obtained a different result in our study. The 17.5°t hreshold for topographic steering into the blowout was considerably less than the 200°and 100°deflection angle suggested by Hesp and Pringle (2001) and Smyth et al (2014), respectively, and was also less than the 50°suggested by Pease and Gares (2013). This may be the most significant difference between the blowouts developed in coastal and inland environments.…”

“…Blowouts are precursors to dune reactivation, and in arid regions dune field reactivation is typically a form of land degradation and is the primary manifestation of desertification (Zhu, 1989;Barchyn and Hugenholtz, 2013), thus they have attracted significant research attention. To date, many studies have focused on the morphology (Catto et al, 2002), airflow patterns (Pluis and Boxel, 1993;Hesp, 1996;Fraser, 1998;Pease and Gares, 2013;Garès and Pease, 2015), erosional and depositional patterns (Bate and Ferguson, 1996;Hesp and Hyde, 1996;Halls and Bennett, 2012) and evolution (Jungerius, 1984;Gares and Nordstrom, 1995;Abhar et al, 2015) of blowouts, especially those developed in coastal dunes. However, there have been few studies of blowouts developed on inland flat grasslands (Hugenholtz and Wolfe, 2006).…”

Influence of topography on wind speed over a coastal dune and blowout system at Jockey's Ridge, NC, USA

“…Previous analysis of the wind data has suggested a relationship between wind speed and wind direction (Pease and Gares, 2013), which this paper examines in greater detail using the wind speed-up ratio. To examine how wind speed-up varies with broad differences in angle of approach, we assign each data entry to one of eight 10°directional categories, ranging from 240°to 320°, based on the regional wind direction recorded at the top of the beach tower.…”

“…Pease and Gares (2013) demonstrated the role of the blowout in modifying the wind direction locally, showing that the regional direction is maintained until the angle between the blowout axis and the direction of wind approach reaches 50°. To facilitate the presentation of results, we have separated the analysis of wind direction patterns from that of wind speed patterns.…”

“…A limited number of studies have examined the influence of wind direction on flow acceleration up the windward slope, showing that the slope is reduced as airflow becomes more oblique to the ridge and consequently acceleration diminishes (Rasmussen, 1989;Hesp et al, 2005;Walker et al, 2009). Carter et al, 1990;Gares and Nordstrom, 1995;Fraser et al, 1998;Hesp and Walker, 2012;Smythe et al, 2012;Pease and Gares, 2013). The primary flow feature in this area is the separation zone where wind speed is significantly lower than on the crest, and air flows as a helicoidal vortex (Sweet and Kocurek, 1990;Walker and Nickling, 2002;Wang et al, 2002;Dong et al, 2008;Lynch et al, 2010).…”